Recently, many kinds of mobile displays have become increasingly popular.
They are widely used under a variety of ambient illumination levels, including bright
outdoor viewing conditions. Manufacturers of mobile displays require new data and
computational models which consider the effect of ambient illumination in order to
develop higher quality products. The aim of this thesis is to analyse the ability of the
human visual system to evaluate the quality of images reproduced on small-size
mobile display media, especially for 2-inch liquid crystal display, and to
computationally enhance quality of images under a wide range of ambient
illumination levels. Specifically, the following four major issues were investigated:
analysis of psychophysical attributes affecting image-quality evaluation,
quantification of image quality using measurable image-properties, measurement of
the ambient-illumination adaptive contrast sensitivity function, and image
enhancement which discounts the effect of ambient-illumination. Since small sized
mobile display was mainly focused on in the current research, the results can be used
for a limited range of display size. Effects 'of display size were remained as future
study.
Firstly, typical variations of mobile display media that exist in the real world
were simulated using a device characterisation technique and psychophysical
attributes (e.g. naturalness, clearness, sharpness, contrastness, colourfulness and
preference) affecting the image quality evaluation process were analysed. The
naturalness and clearness were found to be the most statistically important
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psychophysical-attributes for modelling image quality. As ambient illumination level
increases, the significance of clearness (which is attributed to both contrast and
sharpness) increased and image quality was impaired exponentially.
Secondly, image quality was modelled using the measurable image properties memory
colour reproduction ratio (MCRR), mean chroma and 95th percentile
luminance - regarding to the physical characteristics of display media. A
computational image-colour quality (ICQ) model was developed that consists of
three cognitive processes: local and global assessments and scoring the ICQ. The
effect of outdoor illumination on the ICQ model estimate could' be 'quantified as an
exponential decay function of illuminance (Ix).
Thirdly, to estimate the change in image contrast under a variety of ambient
illumination levels, the contrast sensitivity function (CSF) of the human visual
system was psychophysically measured using the contrast threshold detection
method. As the ambient illumination level increased, the overall contrast sensitivity
decreased and the spatial. frequency where the maximum contrast sensitivity
occurred was shifted towards a lower frequency.
Finally, the loss in contrast sensitivity was quantified and compensated for by
increasing the amplitude of Fourier transform of a given image. An. ambient- .
illumination adaptive weighting function was applied to the' amplitude increase,
. according to the normalised CSF difference between the reference (dark) and a given
target ambient illumination level. This IEA method resulted in a considerable imageÃ‚Â·
quality enhancement under outdoor viewing conditions, as verified through a set of
psychophysical experiments.